1. Field of the Invention
The present invention relates to a display device, such as a plasma display device, which displays pseudo gradation images, and more particularly to a display device which can receive both analog and digital image signals.
2. Description of the Related Art
For example, a digital printer for printing out a gray scale image such as a photograph and painting requires the conversion of a multi-value image signal, inputted from a device such as a scanner or computer for outputting image data, into a signal indicative of gradation in order to support the number of gradation levels of the image print-out mechanism of the printer. Known as a conversion technique mentioned above is the error diffusion technique and the dithering technique.
For example, a variable print-quality digital printer for expressing gradation in a pseudo manner through conversion by the dithering technique is disclosed in Japanese Patent Laid-Open Publication No. Hei 2-266966. The printer disclosed in this publication is adapted to make it possible for a user to select as appropriate between an 8×8 or 4×4 matrix for performing gradation processing, in accordance with the type of the image to be printed out.
In addition, the types of images to be printed out include a character and picture image. If the image to be printed out includes only a character image and the user has selected the 4×4 matrix for performing gradation processing by dithering, the 4×4 matrix would provide less levels of gradation but a higher resolution in the processing than the 8×8 matrix. This allows the character to be printed out clearly. In contrast, if the image to be printed out includes only a picture image and the user has selected the 8×8 matrix for performing gradation processing by dithering, the 8×8 matrix would provide a lower resolution but more levels of gradation in the processing than the 4×4 matrix. This allows a picture smooth in quality to be printed out. Now, suppose the image to be printed out includes both a character and picture image and the user has selected the 8×8 matrix for performing gradation processing by dithering, just as in the case of printing out an image that includes only a picture image. This would provide an image of poorer image quality but of a more balanced character and picture than the cases where the character or picture is independently processed.
In the foregoing, the display device receives mainly an analog signal as the input signal. However, recent years have seen widespread use of displays such as the liquid crystal display and the plasma display, which receive a digital signal as the input signal. If an analog signal is received as the input signal, an analog signal of the RGB color space is also handled even when the display device receives an image signal from a computer. However, recent high-speed transmission technology involves direct input and output of a digital signal between the computer and the display device.
When an analog signal is inputted into the display device as an image signal, the input analog signal is converted into a digital signal by means of an analog-to-digital (A/D) converter. The digital signal outputted from the analog-to-digital converter is subjected to predetermined processing in a picture signal processor. Then, the digital signal is converted by a gradation circuit into a signal indicative of a level of pseudo gradation to support the number of levels of gradation of the display panel. The gradation circuit performs error diffusion processing on the digital signal. In the error diffusion processing, the image signal to be processed is converted to reduce the number of bits thereof, and a quantization error produced upon the conversion is added for diffusion to an image signal to be subsequently processed.
However, there exists noise in the lower bits of the digital signal outputted from the A/D converter. For this reason, when the error diffusion processing is performed in the gradation circuit, the noise acts in the same manner as random numbers. This would cause the error diffusion processing to produce further irregular patterns.
In contrast, when the display device receives directly a digital signal as an image signal, unlike the case of receiving an analog signal, no noise exists in the lower bits of the digital signal. Thus, when image signals of a uniform level of gray scale are inputted successively, this causes a cyclic pattern to be produced after the error diffusion processing has been performed in the gradation circuit.
The pattern to be produced after the error diffusion processing depends on the quantization error of the source of diffusion. Thus, in some cases, this makes it impossible to specify what kind of pattern to be produced, causing the image quality to deteriorate depending on the condition.
Therefore, a digital signal, which has no noise in the lower bits, inputted as an image signal makes it possible to generate a stable pattern by the dithering method when selected as the gradation processing. This therefore ensures the quality of the image.